1. Field of the Invention
This invention relates to display devices of the gas discharge type, generally classified under 315/169.4 of the U.S. Patent Classification System and light sources under H01J61/96 of IPC. In particular, the inventive concept concerns replacing conventional matrix addressing of the glow discharge pixels in gas plasma display panels with electrical pulse transfer of the localized glow discharge along a recurring path with the use of MEMS (Micro Electro Mechanical Systems) and VLSI (Very Large Scale Integration) technology for fabrication. By the scheme greatly simplified pixel addressing for television interlaced scanning is disclosed for a mini plasma display including low fabrication cost achieved by the advantages of MEMS and integrated circuit process technology.
2. Description of the Prior Art
Television Technology
In order for a flat screen to be useful for television it must be able to reproduce the functions of interlaced scanning of the conventional cathode ray tube. One main object of the invention is to duplicate the interlaced scanning requirements for television in a flat screen gas discharge display without the need for separately addressing each of the 1225 horizontal and vertical lines of a conventional matrix display. Another main object is to achieve low cost construction for a plasma screen of the miniature type by use of MEMS and integrated circuit fabrication technology
The invention disclosed must meet the following essential technical criteria for interlaced TV scanning:
The standard picture repetition rate is 30 frames per second to accommodate eye-brain persistence. Interlaced scanning of the CRT electron beam means that first the odd-numbered lines, namely, 1, 3, 5, 7, etc., and then the even-numbered lines, 2, 4, 6, 8, 10, etc. are traced. Accordingly, two fields constitute one frame or a field repetition rate of 60 fields per second. Since 525 horizontal lines is the US standard for television reception and the standard aspect ratio of picture width to height is 4 to 3, and if the spacing between the elements along a horizontal line is the same as that in a vertical direction, then the theoretical maximum number of elements along a horizontal line is 525×4/3, or 700. Since the total number of elements in a picture is equal to the product of the number of horizontal and vertical elements, the theoretical maximum pixel density is 525×700, or 367,500 pixels.
In order to produce a clear steady picture, the scanning operation must satisfy the following requirements: (1) Each frame must be divided into two fields, (2) the rate of forward travel of a horizontal line must be linear, (3) the return trace of a horizontal line must be at a much higher speed than the active trace and should be blanked out, (4) the length of each horizontal line must be the same, (5) the rate of vertical movement of the beam must also be linear, (6) the vertical return trace of the beam must be at much higher speed than the downward motion and should be blanked out, (7) the amount of vertical movement must be the same for each field and each frame, (8) the width of the beam should be equal to the width of one horizontal line, (9) the space between adjacent lines in any field should be equal to the thickness of one line, (10) the first field should trace the odd-numbered lines as 1, 3, 5, etc., until 262½ lines of the 525-line raster are completed, (11) the second field must trace its 262½ lines (2, 4, 6, etc.) in the spaces midway between the lines of the first field, (12) each odd-numbered field must fall in the same position as the preceding odd-numbered field, (13) each even-numbered field must fall in the same position as the preceding even-numbered field.
Transmission of a special synchronizing signal from the television transmitter controls the instant of starting and the length of scanning time for each horizontal line at the picture tube of the receiver and also the vertical motion of the scanning beam. At the end of each horizontal line scan a blanking pulse is provided called the horizontal blanking pulse. The duration of this pulse is 1.27 μsec before the end of the active trace of a line. The sum of both the active and retrace portions for the 525-line system is 63.5 μsec. The duration of the blanking pulses between horizontal lines is approximately 10 μsec. The pulse provided at the end of each field is called the vertical blanking pulse. The duration of the blanking pulses between successive fields is kept within the limits of 1167 and 1333 μsec.
The following functions must be performed during the blanking period associated with the vertical synchronizing pulse: (1) blanking out for a period of 20 to 22 horizontal lines; (2) returning the ‘beam’ from the bottom to the top of the raster; (3) returning the ‘beam’ to either the start of a line or the center of a line so that it will begin the succeeding field at the position required to produce interlaced scanning; (4) continuing the operation of the horizontal oscillator at its proper frequency during this blanking period so that the scanning beam will be at its required position when the blanking pulse ends.
For good television pictures, a video signal with a range of 30 to 4,500,000 cps is desired. The lower frequency values are required when a scene of uniform density or shading is to be transmitted, and the higher frequency values are required for transmission of scenes having a large number of areas of alternately light and dark shading.
X-Y Matrix Plasma Displays
The scanning method presently used in state-of-the-art flat screen displays is matrix or X-Y addressing. Gas discharge lamps operate by passing a high voltage through a low-pressure gas to generate ultraviolet light which then strikes associated phosphors. As these phosphors return to their natural state they emit red, green or blue visible light. Thus they operate like fluorescent lamps, with each pixel the equivalent of a tiny colored bulb. Since plasma display panels (PDPs) are emissive and use phosphor, like CRTs, they have excellent viewing angle and color performance. Plasma display panels (PDPs) are numerous tiny gas discharge lamps of the type described which are individually turned on by use of an X-Y matrix of electrodes. The intersection of a row and column of electrodes at the tiny gas discharge lamp defines a pixel, constituting a tiny source of light. When a voltage is applied to orthogonal electrodes, the gas in the channel becomes ionized and conducts current where the electrodes cross. Within the pixel a gas such as Xenon is converted to plasma form by the electrical voltage applied where the electrodes intersect. The plasma generally emits ultraviolet light activating associated phosphors to cause localized light emission.
Matrix-addressed plasma displays are generally fabricated by forming rows of channels etched into a glass substrate, which are filled with xenon, neon, helium, or combinations of inert gas, then sealed. The gas channels making up the rows of the array are fitted with two electrodes. The electrodes along the rows provide a priming voltage which provides partially ionized gas while perpendicular to the gas channel rows are electrode strips that supply the analog pixel data. Because ionized gas is needed to complete the charging circuit, the column data voltages only have an effect on the pixels in a row for which a plasma channel is partially ionized. Consequently, by electrical activation of separate rows and columns of conductors a picture element is defined at the ‘cross-over’. A matrix plasma display, therefore, operates by addressing a large number of tiny discrete gas discharges in sequence to correspond with the requirements of television scanning. By charging the channel rows in sequence and sending data signals during the time the gas is switched on, the display is addressed row by row. This cumbersome addressing method results in considerable switching complexity.
Another problem with conventional plasma screens is that they have traditionally suffered from low contrast. This is caused by the need to ‘prime’ the cells, applying a constant low voltage to each pixel across a row. Without this priming, plasma cells would suffer the same poor response time of household fluorescent tubes, making them impractical. The pixels, which really should be switched off for proper image contrast, emit some light thus resulting in dull contrast. Kanazawa, et al., describe apparatus attempting to eliminate the reduced contrast and for driving the orthogonal electrodes required of a gas discharge matrix display in U.S. Pat. No. 6,034,482. Tsutomu et al., describe a driving system for matrix operated plasma displays in U.S. Pat. No. 5,995,069. Buzak in U.S. Pat. No. 5,077,553 describes a synchronously addressed driving system for matrix operated plasma display of the X-Y type whereby the orthogonal electrodes are addressed using buffer memory, sample and hold, CCD, or other data drivers schemes.
Since conventional TV consists of 525 horizontal lines at a 4×3 aspect ratio, about 367,500 picture elements must be activated in sequence by matrix addressing. This means that separate connections must be made to 525 rows and 700 columns in the display and their activation must be in accordance with TV scanning requirements. Consequently, X-Y matrix systems are limited by the picture element resolution required for good picture reproduction and by the number of connections required to synchronize addressing 1225 horizontal and vertical connectors. It would be desirable to provide elimination of the considerably complex driving circuitry of matrix driven plasma displays while achieving the interlaced scanning requirements of television. Baasch describes in U.S. Pat. No. 3,681,754 a moving sign plasma display device using multidirectional transfer of the plasma glow by means such as magnetoplasmadynamic propulsion. Gas discharge stepping devices utilizing the bistable nature of the ionization properties of gas discharges are described in U.S. Pat. No. 2,443,407 by Wales wherein 3Ø devices of this type allow transfer of the glow discharge electrode-to-electrode. Townsend in U.S. Pat. No. 2,575,370 describes a special electrode configuration that allows 2Ø operation whereby only two supply lines are required for glow transfer. Witmer describes use of these stepping mechanisms for addressing a flat screen gas discharge display in U.S. Pat. No. 3,532,809.
Liquid crystal displays (LCDD) are another type of flat display. Unlike gas discharge displays, devices of the LCD type have no inherent illumination and consequently they must be backlit. Usually the LCD panels are backlit by fluorescent tubes that snake through the back of the unit and this sometimes results in brighter lines in some parts of the display than others. It would be desirable if the problem of uniform bright backlighting could be solved with preferably a scanning pixel light source.
TV Recurring Raster Patterns
It is well known in the TV art that standards for TV reception and transmission require what is called interlaced scanning by a recurring raster pattern for presentation of a TV image. What this means essentially is that a television display must duplicate the original cathode ray tube scanning method wherein an electron beam scans the screen of the picture tube and the brilliance of the spot produced varies in accordance with the amplitude of the picture-information signal voltage being applied to the control grid of the CRT. If the scanning action at the picture tube in the receiver is in synchronism with the scanning action at the camera tube of the transmitter then the original scene at the transmitter will be reproduced at the receiver. These standard TV requirements are detailed in such books as Essentials of Television, McGraw-Hill, pgs. 20-30 and many others so they will not be detailed here. Raster scanning, as it is called in the industry, requires that any proposed display for TV must be compatible with these requirements, including matrix addressing as described above.
A feature of the invention herein described is that it does not use matrix addressing to meet the scan requirements of TV standards but transfers a gas discharge along gas cavity pixel elements row by row to meet TV raster scan requirements in a manner similar to the scanning method used in the original CRTs.
Microfabrication Technology—Preferential or Selective Etching
As well known in the Microfabrication art (See for example, Fundamentals of Microfabrication, Mark Madow, CRC Press, 2002; pgs 207-280) the fact that silicon can be made crystalline is of extreme usefulness. The art of micromachining is dependent upon crystal plane differences whereby anisotropic etchants “machine,” desired structures in crystalline materials by etching much faster in one crystal plane direction than another. The different crystal planes of semiconductor crystalline materials like silicon have different mechanical and chemical properties for one reason because of differing atomic density. Because of these different properties an important useful characteristic of crystalline silicon is that special etches can be used, called preferential or selective etches (also called structural etches), that exhibit anisotropy. That is, the chosen etch can be made to more quickly etch silicon (or other crystalline semiconductors) in one crystalline direction than the other thereby producing significant difference in etch rate and thus allowing specific desired structures. When carried out properly, anisotropic etching results in geometric shapes bounded by the slowest etching crystallographic planes providing perfectly defined structures. A wide variety of etchants have been used for anisotropic etching of silicon, including alkaline aqueous solutions of KOH, NaOH, LiOH, CsOH, RbOH, NH, OH, and quaternary ammonium hydroxides, with the possible addition of alcohol. Alkaline organics such as ethylenediamine, choline (trimethyl-2-hydroxyethyl ammonium hydroxide), hydrazine and sodium silicates with additives such as pyrocatechol and pyrazine are employed as well. For example, KOH solutions when used to etch <100> crystalline silicon quickly etch the atomic planes in the [100] direction but very slowly etch in the much denser [111] direction resulting in a cavity shaped with precisely defined sides of 54.7 degree slope. Because the cavity is bottomed by crystal planes in the [111] direction the cavity bottom is very slowly etched and thus flat, specular, and mirror smooth. Since lateral mask geometries on planar photoengraved substrates can be controlled with an accuracy and reproducibility of 0.5 μm or better this coupled with the anisotropic nature of preferential etchants allows this accuracy to be translated into control of the vertical etch profile for a silicon cavity. Features of the microfabrication art are used in combination in the present invention to provide the numerous very precisely defined gas cavities required for a miniature plasma display.
Another very important fabrication method used in combination in the present invention to provide numerous very precisely defined gas cavities for a plasma display is MEMS etch-stop technology. The MEMS (Micro Electro Mechanical Systems) art (see The MEMS Handbook, Mohammad Gad-el-Hak, CRC Press, 2002, pgs. 72-73) uses etch-stop techniques based on the fact that anisotropic etchants, especially EDP (Ethylene Diamine Pyrocatechnol), very slowly attack boron-doped (p+) silicon layers compared to non-doped boron layers. Experiments show that the decrease in etch rate is nearly independent of the crystallographic orientation and the etch rate is proportional to the inverse fourth power of the boron concentration. Atomically, the etch rate observed within the etch stop region is determined by the number of electrons available in the conduction band at the silicon surface. This number is assumed to be inversely proportional to the number of holes and thus the boron concentration. It is known in the microfabrication art that the concentration of boron and the depth of a boron layer can be very closely controlled in silicon by use of such well known techniques as diffusion, epitaxial layering, or Si-to-Si bonding. For example, silicon of N-type doping or of light boron doping can be of accurate specific thickness atop an etch-stop layer of silicon of high boron doping. Thereby the top layer of silicon can be etched relatively quickly down to the boron doped layer whereat the etch rate greatly decreases or effectively stops providing a cavity of specular bottom. These desirable features are used in combination in the present invention to provide numerous very precisely defined gas cavities for a plasma display.
The invention herein uses in combination these MEMS and microfabrication techniques such as preferential etch and etch-stop methods to precisely determine the depth of a gas containment cavity whereby extremely accurate gas cavities can be uniformly fabricated. In combination such MEMS technology as anodic bonding of glass to silicon, preferential etching, etch-stop techniques, and preferential tungsten deposition, known in the MEMS art, are used in combination in the present disclosure to provide advantageous construction of a miniature plasma display device.